The present invention relates to a method of manufacturing optical fibers for use in optical communication.
Optical fiber communications are coming into practical use because of reduced transmission losses of optical fibers, improved performance of optical semiconductor elements and progress in other peripheral techniques. The following four methods have been mainly used so far for the production of optical fibers for such optical communications. A first method of manufacturing optical fibers is, as disclosed in Japanese Patent laid-open No. 48-73522 dated Oct. 4, 1973, to cut out of a glass ball a cylindrical body of glass which will serve as the core, to cover the cylindrical body with a glass tube which will serve as the glass clad, and then to hot-draw the cladded structure into an optical fiber. A second method is, as disclosed in Japanese Patent laid-open No. 46-5788 dated Dec. 3, 1971, to form a core glass film on the inner wall of a glass tube, and then to heat and draw the tube thereby to produce an optical fiber having a cross section dense at its very center. A third method is, as disclosed in Japanese Patent laid-open No. 49-10055 dated Jan. 21, 1974 (corresponding to U.S. Pat. No. 3,826,560), to use a mandrel as the starting member, to spray finely pulverized glass around the mandrel, to remove the mandrel, to melt the remaining glass tube by heating to collapse the hollow at its center and to turn it into transparent glass, which is then hot-drawn into an optical fiber. A fourth method is, as disclosed in Japanese laid-open No. 52-121341 dated Oct. 12, 1977 (Japanese patent application No. 51-38883 corresponding to U.S. Pat. No. 4,062,665), to pile up porous glass bodies, which will serve as the core, in an axial direction, then to pile up thereon porous cladding material which will serve as the clad, to melt the cladded core by heating into a transparent glass rod and to hot-draw the rod into an optical fiber.
The first method, wherein the cylindrical core is cut out of a lump of glass and polished, is able to give only optical fibers whose refractive index profile is stepwise but not light focusing fibers having wide-band transmission characteristics. The second method, though a very simple one for the production of low-loss optical fibers, requires a long time to form the glass film and therefore is unsuitable for the manufacturing of optical fibers in large quantities. The third method, wherein pulverized glass is accumulated on a mandrel, is able to produce a large quantity of glass at a time and accordingly is suitable for the mass production of glass fibers. However, when the mandrel used as the starting member is drawn out of the porous glass body, the latter is readily damaged, and therefore this method is unsatisfactory in yield. The fourth method, wherein pulverized glass is accumulated as in the third, is suitable for the mass production of glass fibers. However, since in this method flowing pulverized glass determines the pattern of refractive index profile, the index profile is susceptible to air disturbances, and therefore the method is unsuitable for the steady production of optical fibers having wide-band transmission characteristics and a light-focusing type refractive index profile.